The invention discloses a process for producing a high purity magnesium chloride, which is as follows: the crystal magnesium chloride separated from the salt lake old brine which is evaporated and concentrated or the residual liquid after the crystallization is processed by primary pyrolysis, counter moisture absorption and secondary pyrolysis. The invention also discloses a process for jointly producing the high purity magnesium chloride and the lithium salt, which is as follows: after the salt lake old brine is evaporated, concentrated and crystallized, the obtained residual liquid is processed by dehydration, primary pyrolysis, counter moisture absorption, secondary pyrolysis, gasification and calcination. The invention is widely adapted to the different salt lake brine, the extraction ratio of magnesium chloride and lithium salt is high. The invention is applied to magnesium chloride production alone, and is also applied to lithium salt joint production, thus the presently difficult technology problem of magnesium-lithium separation is completely resolved at home and abroad. The invention has the advantages of low requirement process, simple equipment, simple principle and easy mastery, short process for single product, low cost for comprehensive productions, .The invention is adapted to the production requirement of different size, and provides the new approach for comprehensive development of the salt lake in Chinese western region.